1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a method and system for exercising primary-side control of a switching power converter with feed-forward delay compensation.
2. Description of the Related Art
Many electronic systems utilize switching power converters to efficiently convert power from one source into power useable by a device (referred to herein as a “load”). For example, power companies often provide alternating current (AC) power at specific voltages within a specific frequency range. However, many loads utilize power at a different voltage and/or frequency than the supplied power. For example, some loads, such as light emitting diode (LED) based lamps operate from a direct current (DC). “DC current” is also referred to as “constant current”. “Constant” current does not mean that the current cannot change over time. The DC value of the constant current can change to another DC value. Additionally, a constant current may have noise or other minor fluctuations that cause the DC value of the current to fluctuate. “Constant current devices” have a steady state output that depends upon the DC value of the current supplied to the devices.
LEDs are becoming particularly attractive as main stream light sources in part because of energy savings through high efficiency light output, long life, and environmental incentives such as the reduction of mercury. LEDs are semiconductor devices and are best driven by direct current. The brightness of the LED varies in direct proportion to the DC current supplied to the LED. Thus, increasing current supplied to an LED increases the brightness of the LED and decreasing current supplied to the LED dims the LED.
FIG. 1 depicts an electronic system 100 that converts power from voltage source 102 into power usable by load 104. Load 104 is a constant current load that includes, for example, one or more LEDs. A controller 106 controls the power conversion process. Voltage source 102 can be any type of voltage source such as a public utility supplying a 60 Hz/110 V input voltage VIN or a 50 Hz/220 V input voltage VIN in Europe or the People's Republic of China, or a DC voltage source supplied by a battery or another switching power converter.
The controller 106 provides a pulse width modulated (PWM) control signal CS0 to current control switch 108 in a flyback-type, switching power converter 110 to control the conversion of input voltage VIN into a primary-side voltage VP and secondary voltage VS. The switch 108 is, for example, a field effect transistor (FET). When control signal CS0 causes switch 108 to conduct, a primary-side current iPRIMARY flows into a primary coil 114 of transformer 116 to energize the primary coil 114. When control signal CS0 opens switch 112, primary coil 114 deenergizes. Energizing and deenergizing the primary coil 114 induces a secondary voltage VS across a secondary coil 118 of transformer 116. Primary voltage VP is N times the secondary voltage VS, i.e. VP=N·VS, and “N” is a ratio of coil turns in the primary coil 114 to the coil turns in the secondary coil 118. The secondary-side current iSECONDARY is a direct function of the secondary voltage VS and the impedance of diode 120, capacitor 122, and load 104. Diode 120 allows the secondary-side current iSECONDARY to flow in one direction. The secondary-side current iSECONDARY charges capacitor 120, and capacitor 120 maintains an approximately DC voltage VLOAD across load 104. Thus, secondary-side current iSECONDARY is a DC current.
The load 104 has a certain power demand, and the controller 106 generates the switch signal CS0 in an attempt to cause the switching power converter 110 to meet the power demand of the load 104. Ideally, the power PPRIMARY provided by the primary-side of the switching power converter 110 equals the power PLOAD that is provided to the load 104. However, power losses due to non-idealities in the electronic system 100 result in the power PPRIMARY provided by the primary-side being greater than the power PLOAD delivered to the load 104, i.e. PPRIMARY>PLOAD. To meet the power demand of the load 104, controller 106 utilizes feedback to determine the amount of power actually delivered to the load 104. The controller 106 attempts to generate the control signal CS0 to control the primary-side current iPRIMARY so that the power PPRIMARY meets the power demand of the load 104.
Controller 106 utilizes a feedback control loop to control the power PLOAD delivered to the load 104. To control the power PLOAD, the controller 106 controls the control signal CS0 and thereby controls the primary-side current iPRIMARY. Controlling the primary-side current iPRIMARY controls the primary-side power PPRIMARY provided by the primary-side of the switching power converter 110. The controller 106 adjusts the primary-side current iPRIMARY so that the primary-side power PPRIMARY is sufficient to transfer enough power PLOAD to the load 104 to meet the power demand of the load 104.
To generate the primary-side power PPRIMARY, controller 106 utilizes either secondary-side, feedback-based control via a secondary-side feedback path 124 or primary-side control via sense resistor 126. The secondary-side, feedback path 124 is shown with a ‘dashed’ line to indicate use in the alternative to primary-side feedback. For secondary-side, feedback-based control, the controller 106 senses the secondary current iSECONDARY via the signal iS_SENSE. The secondary-side feedback path 124 generally includes components, such as an opto-isolator or optocoupler, that provide electrical isolation between the controller 106 and the secondary-side of the transformer 110. Since the controller 106 knows the primary-side voltage VP and the turns ratio N, the controller 106 also knows the secondary side voltage VS and knows the secondary-side current iSECONDARY from the feedback signal iS_SENSE. Thus, the controller 106 can directly determine the power PLOAD delivered to the load 104. The controller 106 generates the control signal CS0 to generate the primary-side current iPRIMARY to meet the power demand of the load 104 so that the power demand of the load equals the power provided to the load 104.
The actual peak value of the primary-side current iPRIMARY is directly proportional to the amount of power delivered to the load 104. Thus, for primary-side only control, determination of the actual peak value iPK of the primary-side current iPRIMARY dominates the accuracy of the determination of the amount of power delivered to the load 104. The foregoing statement is especially the case during low power applications since the range of the primary-side current iPRIMARY is reduced. The switch 108 does not turn OFF instantaneously upon detection of a target peak value iPK of the primary-side current iPRIMARY by the controller 106. Once the controller 106 senses that the primary-side peak current iPK_SENSE equals a target peak value iPK and turns switch 108 OFF, the actual primary-side current iPRIMARY has already overshot the sensed peak current iPK_SENSE.
To compensate for the delay in turning switch 108 OFF, the electronic system 100 introduces a feed forward, scaled voltage compensation factor
            V      IN              R      SENSE        ×            R      130                      R        128            +              R        130            to boost the current conducted by the sense resistor 126. RSENSE is the resistance value of the sense resistor 126, R128 is the resistance value of the resistor 128, R130 is the resistance value of the resistor 130. Boosting the current across the sense resistor 126 prior to the controller 106 sensing the primary-side current causes the controller 106 to determine a higher peak current iPK_SENSE that can compensate for the delay in turning off the switch 108. Equation [1] represents the value of the estimated peak current iPK_EST using the fixed, feed forward compensation factor:
                                                        i                              PK                ⁢                                                                  ⁢                _                ⁢                                                                  ⁢                EST                                      =                                                            i                                      PK                    ⁢                                                                                  ⁢                    _                    ⁢                                                                                  ⁢                    SENSE                                                  +                                                                            V                      IN                                                              R                      SENSE                                                        ×                                                            R                      130                                                                                      R                        128                                            +                                              R                        130                                                                                                        =                                                i                  PK                                +                                                                            V                      IN                                        L                                    ×                                      t                    DELAY                                                                                ;                ⁢                                  ⁢        and                            [        1        ]                                          t          DELAY                =                              L                          R              SENSE                                ×                                                    R                130                                                              R                  128                                +                                  R                  130                                                      .                                              [        2        ]            iPK_EST is the estimated peak value of the primary current iPRIMARY, and iPK_SENSE is the sensed peak value of the primary-side current. As previously stated, RSENSE is the resistance value of the sense resistor 126, R128 is the resistance value of the resistor 128, R130 is the resistance value of the resistor 130, L is the inductance value of the primary-side coil 114, and tDELAY, as defined by Equation [2], is the delay due to the switch 108 OFF. Since the compensation factor
            V      IN              R      SENSE        ×            R      130                      R        128            +              R        130            tracks well with the input voltage VIN, for a given inductance value L of the primary-side coil 114, the compensation factor
            V      IN              R      SENSE        ×            R      130                      R        128            +              R        130            effectively cancels out delays in turning the switch 108 OFF.
However, secondary-side sensing requires additional, potentially relatively expensive components. Using primary-side sensing and applying the compensation factor
                    V        IN                    R        SENSE              ×                  R        130                              R          128                +                  R          130                      ,which equals
                    V        IN            L        ×          t      DELAY        ,works for a particular inductance value L of the primary-side coil 114. However, the inductance value L of the primary-side coil 114 can vary from transformer to transformer by, for example, at least +/−10%. Thus, if the inductance value L used by the controller 106 differs from the actual inductance value L for the primary-side coil 114, then the estimation of the peak value of the primary-side current iPRIMARY can result in errors providing power to the load 104. Additionally, altering the primary-side current value across the sense resistor RSENSE prior to sensing a representative value of the primary-side current iPRIMARY utilizes external components, which increase the cost of the electronic system 100.